In the manufacture of cigarettes on modern, high speed cigarette makers, extreme care is taken to assure that the cigarette rod is held to a specified firmness or density. If the cigarette rod is too dense, excessive tobacco will be used and the resulting cigarette may not have the desired smoking characteristics such as draft resistance, puff count, etc. If too little tobacco is used, the resulting cigarette will be undesirably soft, tobacco may fall out of the end of the rod, and as is the case with too dense a cigarette rod, the smoking characteristics will vary from the designed parameters.
Modern cigarette machines produce cigarettes at speeds approaching 8000 cigarettes per minute and an attempt is made to continuously monitor the density of the cigarette rod at that speed. Numerous pneumatic devices have been suggested such as those disclosed in U.S. Pat. Nos. 3,411,513; 3,595,067, and 3,850,029 and in British Pat. No. 1,372,056. These devices are (1) of the type employing floating nozzles issuing pressurized air onto the cigarette wrapper and either measure deformation of the wrapper or displacement of the nozzle based on the change in back pressure or (2) of a type providing a pressurized on-line chamber arrangement and observing pressure changes caused by variation of firmness or dimensions of cigarettes passing therethrough.
Alternate, non-pneumatic approaches to firmness measurement are set forth in U.S. Pat. No. 2,667,172 and South African Patent Application No. 73/9394. These devices suggest the use of tobacco rod forming elements of existing machinery for the additional function of providing output indication of rod firmness. In U.S. Pat. No. 2,667,172, an elongated short tongue is provided with a strain gauge positioned proximate its compression foot and a second gauge at the short tongue support clamp or beam. These gauges are in spaced longitudinal alignment whereby longitudinal pressures exerted by tobacco against the tongue may be sensed. This sensing apparatus is separate in function and operation from the tobacco rod firmness sensing apparatus of the South African Application. The latter apparatus comprises a single tongue support beam and foot adapted to provide output indication of vertical strain placed on the short tongue, i.e. movement of its foot transverse to the direction of movement of tobacco engaged thereby. The South African patent application relates also to specially constructed short tongues, disclosing a first embodiment wherein the compression foot of a short tongue is split into two successive longitudinal sections, each having a separate support flange with one support flange having a strain gauge thereon, and a further embodiment wherein the front compression foot section is further split into three circumferential segments, each having an independent support flange with a strain gauge thereon. Such support flanges are stems having one end terminating at the compression foot and an opposite end terminating at the tongue cantilever support beam.
A more recent advance to measure the firmness or density of the tobacco rod in a cigarette maker is to subject the formed tobacco rod to the radiation of beta or other suitable rays which are absorbed by the material in known proportion to its mass, and to determine the absorption by an ionization chamber. This system is disclosed in U.S. Pat. No. 2,704,079.
Once a signal is produced, either by a pneumatic sensor, a strain gauge type sensor or a beta ray type gauge, it can be suitably manipulated for feedback control of the amount of tobacco in the rod and therefore the cigarette rod density. This is normally accomplished by using the signal to control an ecreteur knife motor to adjust the position of the ecreteur knife relative to the tobacco supply vacuum belt. This belt is normally provided with excess tobacco and the ecreteur knife is utilized to slice off the excess tobacco to provide an accurate quantity of tobacco to the cigarette rod forming portion of the cigarette machine and produce rods of the desired density.
Besides sensing the firmness of the finished rod, a further refinement to more accurately control rod density employs a second, pneumatic sensor adjacent the tobacco vacuum feed belt to determine the density of the tobacco on the belt upstream from the ecreteur knife. The feed-forward signal form this second sensor is used in conjunction with the signal from the rod density control to control the position of the ecreteur knife.
A cigarette maker presently available which uses two input signals for weight control is offered by Molins Limited. The "Modic" controller on this machine uses the first signal in a threshold type feedback system such that when the signal from a .beta. gauge indicates a rod density error of more than 1/2%, the ecreteur knife motor is energized in the proper direction to bring the error to within .+-.1/2% of the desired density. The ecreteur motor operates at a set speed and therefore responds at the same rate to correct minor or major error signals. The second signal (pneumatic sensor) drives a separate hydraulic positioner with a bellowscontrolled valve to provide more rapid, gross positioning of the ecreteur knife in a feed-forward arrangement.
A second feedback system, currently in use, is the "Accuray" Controller manufactured by Accuray Corporation. This system utilizes a digital computer to provide integral and proportional processing of the signals from a .beta. gauge to provide a loop control output signal for controlling the ecreteur motor and thereby provide a system with no net error. The "Accuray", like the Molins Modic, may also be employed in conjunction with a second, pneumatic, sensor.
However, these systems are not completely satisfactory in that the ecreteur knife positioning motor responds to the control signals at a fixed rate irrespective of the degree of error in rod density. Furthermore, there is no provision to automatically tune the feedforward systems to compensate for process changes. At the high output speeds of current cigarette machines, this can result in excessive production out of specifications before the error is corrected.